1. Field of the Invention
The present invention relates to a range finder suitable for the use in, for example, an automatic focussing camera.
2. Description of the Prior Art
FIG. 1 in the accompanying drawings shows the measurement principles of the range finder of the kind referred to. Numeral 1 denotes an object, 2 and 3 denote a pair of lenses, and 4 denotes a focal plane of the lenses 2 and 3. Numerals 5, 6 and 7 denote images, and 8 and 9 denote a first and a second light receiving element trains.
The object 1 forms images 5 and 6 on the focal plane 4 respectively through the lenses 2 and 3. If the object 1 is positioned at infinity, the light from the object 1 incides along a light path l and forms an image 7 on the focal plane 4. Accordingly, if the distance x between the images 6 and 7 is detected, the distance a between the object 1 and focal plane 4 can be obtained from the wellknown trigonometry as follows: EQU a=f.multidot.B/x (1)
Here f is the focal length of the lenses 2 and 3 and B is the base line length. In order to obtain clear images on the focal plane 4, f is selected to be f&lt;&lt;a.
In obtaining the value x in the expression (1), the image 5 formed through the lens 2 is used instead of the image 7 of the object at infinity formed through the lens 3. In more detail, the first and second light receiving element trains 8 and 9 are arranged in the vicinity of the image forming positions on the focal plane 4, and the difference between the distance between the image patterns of the images 5 and 6 which are obtained by these light receiving element trains 8 and 9 and the base line length B is adopted as the value x.
In FIG. 1, for easy understanding of the measurement principles, the optical system and light receiving element trains are arranged to form a right-angled triangle with the corner at the image 5 right-angled. In fact, it is actually possible to position the object 1 in various other positional relationships with respect to the optical system and light receiving element trains. In any event, it makes no substantially changes whether the position of the object 1 is in front of one of the light receiving element trains or on either side thereof. For example, the object 1' forms images on the focal plane 4 at positions displaced from the both ends of the base line length B by x.sub.1 and x.sub.2 respectively. Accordingly, if the x is replaced by x.sub.1 +x.sub.2 in the expression (1), the distance a can be obtained by the similar procedure.
The structure illustrated in FIG. 1 will now be described in detail. FIGS. 2(a) and 2(b) are block diagrams which show the practical structure for obtaining the distance x.
Referring to FIG. 2(a), numerals 8 and 9 denote the light receiving element trains as illustrated in FIG. 1, numerals 10 and 11 denote binary-coding circuits for binary-coding the output from the light receiving element trains 8 and 9, and numerals 12 and 13 denote shift registers of the same bit number as the light receiving element trains 8 and 9. Numeral 14 denotes a coincidence detection circuit train, 15 denotes a counter, and 16 denotes a discrimination circuit. The respective light receiving elements in the light receiving element trains 8 and 9 provide output in the analogue form, which is binary-coded by an appropriate threshold level in the binary-coding circuit trains 10 and 11, and then stored in the shift registers 12 and 13. In this instance, it is not always necessary to form the both 12 and 13 shift registers. As shown in FIG. 2(b), if one of them, 12 in the illustrated example, is formed to be a latch circuit, the other, 13 in this example, may be desirably constituted by a shift register.
Referring to FIG. 2(a) again, the outputs from the respective bits of the shift registers 12 and 13 are applied on into the coincidence detection circuit train 14 in the preset combination. Each circuit in the coincidence detection circuit train 14, on detecting the coincidence between the two inputs, makes "1", otherwise on detecting no coincidence therebetween, makes "0".
The outputs "1" are countered by the counter 15 and the counted number is supplied to the discrimination circuit 16. The discrimination circuit 16, after storing the counted number, makes the shift register 12 and/or 13 to shift by one bit and read the output from the counter 15, which is stored by memory means (not shown). This series of shifting of the shift registers 12 and 13 and reading/memorizing of the counted number on the counter 15 is repeated by the predetermined number. Then the largest value in the counted number stored in the memory means is obtained. This value indicates the case where the images by the light receiving element trains 8 and 9 are most coincided. The number of the shiftings of the shift registers 12 and/or 13 from the initial state to the maximum coincidence number represents x.
It should be noted that the number of bits of the coincidence detection circuit is not necessarily identical with the number of the elements of the respective light receiving elements. Further, if the respective light receiving elements do not have the same number of elements, the shifting number x can be obtained by the similar procedure only by making a minor modification to the circuit structure. For example, by comparing only a part of the shift register in accordance with the arrangement of the optical system and light receiving elements, or in the right-angled type by a successive comparison while shifting only one of the shift registers with the other supplied with no shift pulses or while supplying shift pulses to the both shift registers alternately.
A conventional circuit for binary-coding the output from the light receiving elements is illustrated in FIG. 3. In this figure, only one light receiving element is exemplified for easy understanding. If a predetermined number of light receiving elements of this type are provided, a corresponding circuit to the light receiving element train can be formed. Numeral 17 denotes a photodiode which constitutes the light receiving element, numerals 18 and 19 denote switching transistors, 20 denotes a capacitor and 21 denotes an inverter.
In the operation, at first the switching transistor 18 is turned ON by a CLEAR input to make the capacitor C to discharge. After that, the switching transistor 18 is turned OFF by the CLEAR input and the switching transistor 19 is turned ON by D input. Then photocurrent i which is substantially proportional to the strength of the light flows into the capacitor 20 from the photodiode 17 through the switching transistor 19. When a predetermined time period t has elapsed after the turning ON of the switching transistor 19, the switching transistor 19 is turned OFF by the D input. At this time the capacitor 20 has electric charge of about i.times.t, therefore the input of the inverter 21 is applied with voltage V.sub.in =i.multidot.t/C. Representing the threshold voltage of the inverter 21 by V.sub.th, when V.sub.in .gtoreq.V.sub.th, the output value of the inverter 21 becomes "0", and when V.sub.in &lt;V.sub.th, the output value of the inverter 21 becomes "1".
Assuming that the power supplying time period of the switching transistor 19 is too long, the capacitor 20 in all the light receiving elements is overcharged so that the input to the inverter 21 exceeds the threshold voltage. To the contrary, assuming that the power supplying time period is too short, the capacitor 20 is charged only insufficiently so that the input to the inverter 21 cannot exceed the threshold voltage. That is, if the time t is not taken into consideration at the time of binary-coding, only one pattern that all output values are "0" or "1" can be obtained. Accordingly, the distance measurement is impossible.
Thus the time t should be determined in consideration of the amount of light accepted by the light receiving element trains, and this kind of control is generally complex. In addition, in the comparison by the binary-coded patterns, correctiveness is apparently lacked due to the shortage of information. However, if the multiplication of information is to usually realized through the A/D conversion of photocurrent, it results in the increase in the manufacturing costs and operation time.
For the above reasons, the conventional range finder of this kind cannot perform a correct distance measurement for a relatively short time period.